The present invention relates to optical communication systems and, more particularly, to amplitude modulation of a pump laser signal in a distributed Raman amplifier.
Optical communication systems use optical fiber as a communication medium and light as an information carrier. For instance, an optical signal may be a beam of light modulated to represent binary-coded information. When light is used to transmit information, the information may be extracted from the beam of light through the use of a photodetector in a receiver. A photodetector is an electronic component that detects the presence of light radiation through conversion of light energy to electrical energy. A common photodetector is called a photodiode which consists of a semiconductor having a property called photoconductivity, in which the electrical conductance varies depending on the intensity of radiation striking the semiconductor material comprising the photodiode. Essentially, a photodiode is the same as an ordinary diode, except that the package has some transparency that allows light energy to effect junctions between the semiconductor materials inside.
To make efficient use of a single optical fiber, many unique data signals may be transmitted over the same fiber so long as each data signal modulates an optical signal with a wavelength different from the other optical signals on the same fiber. When the wavelengths of the different optical signals are only marginally different from one another, the transmission scheme may be called Dense Wavelength Division Multiplexing (DWDM). In a network using DWDM, two elements connected by a single physical link (optical fiber) may communicate using a number of signal channels, where each signal channel is an optical signal with a distinct wavelength.
One detrimental factor to be considered in optical communication is attenuation. Essentially, signal power is attenuated over the length of a fiber such that the power of a signal at the receiving end of a transmission link is less than the power of the signal at the sending end of the transmission link. To overcome attenuation, a long optical transmission link may be made up of several shorter spans. At a repeater at the beginning of each span after the first, the optical signal is received and subsequently regenerated at a power level that will allow accurate reception at the end of the span. Unfortunately, these repeaters require that the optical signal be converted to an electrical signal and this conversion has several inherent drawbacks.
Fortunately, equipment that amplifies the optical signal without converting it to an electrical signal has been developed, one example of which is the Erbium Doped Fiber Amplifier (EDFA). The idea behind an EDFA is that if a fiber is doped (enhanced) with Erbium, and a signal from an external light source (for instance, a laser) is used to excite the Erbium, the power of an optical signal passing through the fiber will be increased.
Rather than amplify an optical signal in an amplifier, distributed amplification is possible wherein part of the transmission fiber acts as an amplifier. One such distributed amplification scheme is called xe2x80x9cRaman amplificationxe2x80x9d wherein high-power laser light is sent (or pumped) on the same transmission fiber as the data signal. Raman amplification is named after the scientist who discovered a phenomenon in the scattering of light, called the Raman Effect, in 1928. Raman amplification takes advantage of stimulated Raman scattering, which occurs in a silica fiber when an intense pump laser beam propagates through it. Stimulated Raman scattering is an inelastic scattering process in which an incident pump photon loses its energy to create another photon of reduced energy at a lower frequency. That is, pump energy of a given wavelength amplifies, or provides gain to, a signal at a longer wavelength. To date, distributed Raman amplification is an open loop process where the pump powers, but not the gain, may be controlled.
Various measures of quality exist for measuring the operation of optical systems. The term xe2x80x9cQxe2x80x9d may used to describe the xe2x80x9cqualityxe2x80x9d of a particular optical transmission system in terms of the eye opening when a digital signal is transmitted over the particular optical transmission system. Additionally, the term xe2x80x9cOSNRxe2x80x9d may be used to describe an optical signal-to-noise ratio present in a given optical system.
For a particular span, the qualities of the transmitter, the medium and the receiver may be summarized in a single number, called a xe2x80x9clink budgetxe2x80x9d. The link budget represents the amount of allowable attenuation of a signal over the span, say 30 dB. If an ancillary component is included in a span, for instance for service channel transmission, additional attenuation may be introduced. If a component reduces signal power by one dB, the link budget would then be 29 dB. The one dB reduction in link budget is called a xe2x80x9clink budget penaltyxe2x80x9d.
With knowledge of the traffic signal at the input and output of a distributed Raman amplifier, the gain of the amplifier is relatively straightforward to determine. However, often the traffic signal is not available to the operator of such an amplifier. Clearly, it would be desirable to have closed loop control of the Raman amplification process in the transmission fiber in the absence of access to the input signal.
A pump laser signal in a distributed Raman amplifier, which includes a length of transmission fiber and a pump laser, is modulated with a small amount of amplitude modulation. This amplitude modulation transfers to the traffic wavelengths and is detected at a Raman amplifier control system. The Raman amplifier control system, given knowledge of the received modulation and the applied modulation, may determine the gain of the distributed Raman amplifier. The Raman amplifier control system may then control the power supplied to the pump laser to result in a specific gain, thus providing closed loop gain control. Control of the gain of the distributed Raman amplifier results in more accurate control of the signal at the output of the distributed Raman amplifier. Advantageously, more accurate control of the output signal level leads to improved optical link budgets, which translates into better control of Q or OSNR.
In another aspects of the invention, an increase in determined gain may allow a determination of any increase in reflection caused by optical components at the far end of the length of transmission fiber. Further, the amplitude modulation may allow a receiver to identify a particular pump laser signal. This may assist in a determination of a power loss over the length of the optical transmission fiber.
In accordance with an aspect of the present invention there is provided a method for facilitating determination of a characteristic of an optical signal on an optical fiber, including amplitude modulating a pump laser signal of a distributed Raman amplifier.
In accordance with another aspect of the present invention there is provided a method for facilitating determination of a characteristic of an optical signal on an optical fiber. The method includes identifying a particular pump laser signal of a distributed Raman amplifier from amplitude modulation of the particular pump laser signal, measuring a power of the particular pump laser signal, receiving an indication of transmitted power in the particular pump laser signal and determining a power loss from the indication of transmitted power and a power determined from the measuring the power of the particular pump laser signal.
In accordance with a further aspect of the present invention there is provided a method of monitoring gain in a distributed Raman amplifier, where the distributed Raman amplifier includes a length of transmission fiber and a pump laser. The method includes receiving an indication of amplitude modulation applied to a pump laser signal of the distributed Raman amplifier, receiving an indication of amplitude modulation output from the distributed Raman amplifier and determining a gain from the indication of amplitude modulation applied and the indication of amplitude modulation output. In another aspect of the invention a gain monitoring system and a pump power controller are provided for performing this method. In a further aspect of the present invention, there is provided a software medium that permits a general purpose computer to carry out this method.
In accordance with a still further aspect of the present invention there is provided a method of controlling gain in a distributed Raman amplifier, the distributed Raman amplifier includes a length of transmission fiber and a pump laser, the method includes generating a pump laser controlling current, amplitude modulating the pump laser controlling current to create a modulated pump laser controlling current and outputting the modulated pump laser controlling current to the pump laser of the distributed Raman amplifier. In another aspect of the invention a gain controller for a distributed Raman amplifier is provided for performing this method. In a further aspect of the present invention, there is provided a software medium that permits a general purpose computer to carry out this method.
Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.